Document SaveFileDialog.OpenFile() failure behvaior on various platforms.

On Mono, [any behavior which would result in an exception returns `null`](https://github.com/mono/mono/blob/4b5e2591eb299eb34d60b5217b14c1ad9f11e238/mcs/class/System.Windows.Forms/System.Windows.Forms/SaveFileDialog.cs#L88), making it [inconsistent with .netfx](https://github.com/mono/mono/issues/21455).

On [.netfx](https://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/SaveFileDialog.cs,66a104767467d192) and [.netcore](https://github.com/dotnet/winforms/blob/c402e2a21cc3d6ab1c3fe6209ff829361aa596b9/src/System.Windows.Forms/src/System/Windows/Forms/SaveFileDialog.cs#L77), it is a thin wrapper around `new FileStream()`, so any exception which that constructor throws may be thrown. A `null` reference cannot be returned.

The [existing documentation](https://docs.microsoft.com/en-us/dotnet/api/system.windows.forms.savefiledialog.openfile?view=netframework-2.0) has no discussion about how errors are handled. It should at least mention:

* If the dialog was not shown with a successful result prior to invocation, the method will throw `ArgumentNullException`.
* If the file cannot be opened (such as if a race condition results in the file being moved), one of the following may happen:
* `IOException` thrown (on .netfx/.netcore).
* `null` may be returned.